In positioning systems based on satellites, the positioning receiver attempts to receive signals transmitted from satellites, to compute its own position. Normally, the reception of at least four different satellite signals is needed to determine the three-dimensional position and the time data. However, satellite signals are relatively weak upon arrival at the receiver. The signal level is often below the level of background noise. In practice, this means that the positioning receiver cannot necessarily receive the required number of satellite signals. For such a situation, there are systems under development which utilize assistance information and mobile communication networks. In this case, assistance information related to the satellites is transmitted to the positioning receiver via base stations of a land mobile communication network. In such a system, at least some base stations are provided with a positioning receiver to receive signals transmitted from satellites. At the base station, the coordinates of the base station are known at a relatively high precision, wherein the satellite signals received at the base station and the coordinates of the base station can be used to determine some information relating to the satellite signals, such as the propagation time from the satellite to the base station and the ionospheric corrections. Furthermore, it is possible to transmit assistance information, such as Ephemeris and almanac data, from an earth station belonging to the satellite positioning system to the base stations. In this case, the assistance information can be utilized in the positioning to use also such a satellite signal whose strength is not sufficient for the demodulation of the information in the signal. This will facilitate the positioning under poor signal conditions.
One known positioning system based on satellites (satellite positioning system) is the Global Positioning System (GPS) comprising several satellites orbiting the earth. Each operating satellite of the GPS system transmits a so-called L1 signal at the carrier frequency of 1575.42 MHz. This frequency is also indicated with 154f0, where f0=10.23 MHz. Furthermore, the satellites transmit another ranging signal at a carrier frequency of 1227.6 MHz called L2, i.e. 120f0. In the satellite, these signals are modulated with at least one pseudo random sequence. This pseudo random sequence is different for each satellite. In each satellite, for modulating the L1 signal, the pseudo random sequence used is e.g. a so-called C/A code (Coarse/Acquisition code), which is a code from the family of the Gold codes. Each GPS satellite transmits a signal by using an individual C/A code. The codes are formed as a modulo-2 sum of two 1023-bit binary sequences. The first binary sequence G1 is formed with the polynomial X10+X3+1, and the second binary sequence G2 is formed by delaying the polynomial X10+X9+X8+X6+X3+X2+1 in such a way that the delay is different for each satellite. This arrangement makes it possible to generate different C/A codes by using identical code generators. The C/A codes are thus binary codes whose chipping rate in the GPS system is 1.023 Mchips/s. The C/A code comprises 1023 chips, wherein the iteration time (epoch) of the code is 1 ms. The carrier of the L1 signal is further modulated by navigation information at a bit rate of 50 bit/s. The navigation information comprises information about the “health”, orbit, time data of the satellite, etc. In the GPS system, the codes used in the modulation of the L1 signal are not particularly efficient in view of eliminating the above-presented narrow-band interference. Thus, the cross-correlation caused by a strong spurious signal in the weaker signal to be received may prevent the receiver from acquiring this signal to be received.
International patent application WO 99/47943 presents a system and a method for determining the position of a wireless CDMA transceiver. The publication presents typical transmission of assistance information to a receiver in the case of satellite positioning. The assistance information can be satellite identification data, Dopper shift data, information related to the distance between the base station and the satellite, or information related to a search window. The search window can be determined on the basis of the round trip delay between the receiver and the base station, or information related to the angle of the satellite. In the system according to the reference, the base station is provided with a GPS receiver which the base station can use to acquire information relating to satellite signals, for example to determine the distance between the satellite and the base station.
One problem in systems of the above-described kind is that the implementation of the system requires modifications in the mobile communication network, for example for the reason that at least some base stations must be provided with a positioning receiver. Furthermore, the system is bound to the communication protocol used on the transmission channel of the mobile communication network, and the implementation of the physical layer. Also, the operator of the mobile communication network must implement the transmission of the assistance information in the mobile communication network. Furthermore, the use of the system requires that the user's electronic device is provided not only with the positioning receiver but also a receiver functioning in such a mobile communication network. Consequently, it is a system bound to a specific communication network and protocol. The implementation of such a system is relatively expensive, and the use of the system is limited to the subscribers of this mobile communication network only.